The C# HttpClient doesn't add a User-Agent header by default. The CDN provider (Cloudflare in this case) checks for the presence of a User-Agent header to distinguish somewhat between regular browsers and programmatic access.
Just add a User-Agent to your HttpClient and make sure it identifies you.
Identify yourself. Add something useful to the user-agent (ideally, a link to an URL, or something informational) so we can see your bot as something other than "generic unknown anonymous scraper."
See also: